In image forming devices employing an ink-jet system (referred to as “ink-jet printer” hereinafter where appropriate), improvement in dot forming method is sought for to improve image quality and reduce drying time.
For example, U.S. Pat. No. 5,596,355 (publication date: Jan. 21, 1997) discloses a technique of forming an image with the use of a slow-drying black ink, having high black reproducibility but slow drying time, and a fast-drying black ink, which dries fast but its print density is low. In this technique, when color dots are formed adjacent to the area where black dots are formed, the fast-drying black ink is used, or inks of C, M, Y are overlaid to make up the boundary area, and the slow-drying black ink is used to form the other area.
This improves reproducibility of black and suppresses mixing of black dots and color dots at the boundary.
Further, Japanese Unexamined Patent Publication No. 149036/1995 (Tokukaihei 7-149036) (publication date: Jun. 13, 1995) discloses a technique which uses a black ink which has low permeability with respect to a recording sheet, and inks of C, M, and Y which have high permeability. FIG. 37 and FIG. 38 show an example of how dots are formed by this technique. That is, when a color dot area is formed adjacent to a black dot area, the black dots in the black dot area are interpolated and color dots are formed instead therein (staggered dots are formed). Further, it also teaches forming color dots as an underlying layer of a black dot area so that the black dots are formed over on the color dots.
This is intended to prevent mixing of the black dot area and the color dot area and to reduce a drying time of the black dots.
Further, Japanese Unexamined Patent Publication No. 197831/1996 (Tokukaihei 8-197831) (publication date: Aug. 6, 1996) discloses a technique similar to that of the foregoing publication No. 7-149036.
Further, Japanese Unexamined Patent Publication No. 338136/1993 (Tokukaihei 5-338136) (publication date: Dec. 21, 1993) discloses finding a proportion of black dots in an image to be formed and an ambient temperature of image formation, and changing a transport speed, etc., of a recording sheet with an ink based on the proportion of the black dots and the ambient temperature thus found, so as to ensure drying of the ink on the recording sheet.
Also, apart from the improvement in print method as above, there have been many proposals for reducing drying time of prints by the provision of drying means which performs heating using a halogen lamp, for example.
However, in the technique disclosed in the foregoing U.S. Pat. No. 5,596,355, the following problems are caused in forming a high black dot density area (referred to as “solid black area” hereinafter).
That is, in this technique, because the solid black area is formed with the slow-drying black ink having high reproducibility of black to improve image quality, there are cases where the prints are contaminated or black is seen through the back of the sheet due to the black ink which has not been dried sufficiently. This is due to a correlation between black dot density and drying time, whereby the drying time becomes longer in a solid black area where the black dot density is high, as in characters of 10 points or larger, or lines of 0.5 point or larger, and which exceeds a certain area.
In particular, in ink-jet printers which employ the face-down system to improve operability, the problem of sticking ink to the transport roller, which first comes into contact with the print surface immediately after the print process, and the problem of re-transfer of an ink to the recording sheet become more pronounced.
On the other hand, when the fast-drying black ink is used to form the solid black area, image quality suffers because reproducibility of black is poor.
Further, the foregoing publication No. 7-149036 and No. 8-197831 have the problem of poor image quality of black due to color dots, i.e., due to co-existing monochromatic color of yellow (Y), magenta (M), or cyan (C) in a boundary area in the black dot area.
Further, the foregoing publications do not disclose reducing drying time in a high black dot density area.
Further, according to the technique disclosed in the foregoing publication No. 5-338136, printing is made, taking into consideration black dot density and ambient temperature. However, this technique merely adjusts the drying time based on black dot density and ambient temperature, and the image forming rate may slow down depending on the image to be formed or ambient temperature. That is, this technique is not intended to actively reduce the drying time.
Therefore, this technique is bound to the problem of print contamination and see-through of black due to insufficient drying when the solid black area is formed using the slow-drying black ink to improve image quality while, at the same time, maintaining the image forming rate. Further, when these problems are to be solved by the foregoing technique, the drying time becomes longer under low-temperature conditions where ink viscosity is increased, and as a result recording speed becomes slow.
As described, the foregoing techniques of the prior art have various problems which are associated with drying of a solid black area in image formation.
On the other hand, the technique which provides the drying means has the problem of complex device structure and increased power consumption due to power consumed by the drying means.
Further, in ink-jet printers, generally, images are formed successively with respect to recording sheets which are continuously fed, and the recording sheets with images are successively discharged to a discharge tray and stacked thereon. In this case, in the event where subsequent recording sheets are discharged while the ink on the preceding recording sheet which was discharged previously has not been dried completely, there will be contamination of images due to contact between the recording sheets. In view of this problem, various proposals have been made to improve image forming operation, so that subsequent recording sheets are stacked after the ink on the preceding recording sheet is completely dried.
For example, the foregoing publication No. 5-338136 discloses a technique of calculating a black pixel ratio in an image to be formed and finding an ambient temperature of the device, and changing the transport speed of the recording sheet, which has been applied with an ink, based on the calculated black pixel ratio and the detected ambient temperature of the device, so as to ensure that the ink is dried on the recording sheet on the discharge tray before subsequent recording sheets are discharged.
Further, Japanese Unexamined Patent Publication No. 9-76591 (publication date: Mar. 25, 1997) discloses a technique of measuring the time required to dry the ink on a recording sheet which was discharged previously, and the elapsed time from the end of discharge of this recording sheet, so as to carry out intermittent transport operation of subsequent recording sheets in such a manner that the elapsed time exceeds the time required to dry the ink.
Further, Japanese Unexamined Patent Publication No. 5664/1999 (Tokukaihei 11-5664) (publication date: Jan. 12, 1999) discloses a technical idea wherein a discharge stacker is adapted to have a discharge support of plural stages, and recording sheets having been formed with images are replaced one after another in the stages of the discharge support, so as to delay the time of contact such that the recording sheets come into contact with each other after the ink has been dried.
However, in the technique disclosed in the foregoing publication No. 5-338136, while it takes into consideration black pixel ratio and ambient temperature of the device, it merely adjusts the drying time based on these variables. Thus, there were cases where the image forming rate slowed down depending on the image to be formed or ambient temperature of the device. Particularly, when the ambient temperature of the device is low, the image forming rate is decreased greatly. That is, this technique is not intended to actively reduce drying time of the ink.
Similarly, the technique disclosed in the foregoing publication No. 9-76591 is also for increasing the time required to form an image on a subsequent recording sheet, and there were cases where the image forming rate was decreased greatly depending on the image to be formed. That is, this technique is not for actively reducing drying time of the ink either.
Further, in the technique disclosed in the foregoing publication No. 11-5664, not only the structure of the discharge stacker is made complex but it requires a driving power to replace the recording sheets one after another in plural stages of the discharge support, and as a result power consumption of the entire image forming device may be increased.
As described, none of the foregoing prior art realizes stacking subsequent recording sheets after the ink on the previously discharged recording sheet is completely dried, without increasing the time required to form an image and without resulting in a complex discharge structure of the device.
Further, even though there have been proposals as above to provide drying means such as a heater to facilitate drying of the ink, this is not practical since it results in complex device structure and large power consumption by the drying means.